WASHINGTON — As Democrats proved during the 2006 and 2008 campaigns, the fastest way for a party to make big gains is to contest and win districts where it hadn’t seriously competed in many years.

Now the comeback-minded Republicans are probing a similar strategy for 2010, by contesting districts long held by Democratic incumbents — such as 17-term Missouri Rep. Ike Skelton.

After cruising for many years over little-known opponents in west-central Missouri’s 4th District, Skelton appears certain to face a challenge next year from a Republican with a track record of success in state legislative elections. Both state Sen. Bill Stouffer and former state Rep. Vicky Hartzler have confirmed that they are candidates for the GOP nomination.

This, in turn, has prompted CQ Politics to change its rating on the race from “Safe Democratic,” which denotes a non-competitive election, to “Democrat Favored,” which means there is at least a possibility that a highly competitive race may develop.

This rating change hardly means that Skelton is in dire trouble at this juncture. The low-key 77-year-old incumbent received 66 percent of the vote in his 2008 contest and 68 percent in 2006, typical showings for his long career. The only time in his 16 re-election campaigns that he received less than 60 percent of the vote was in a 1982 race against Republican Rep. Wendell Bailey, an incumbent-incumbent face-off induced by redistricting.

Skelton has voted more consistently with the House Democratic leadership than usual over the past couple of years. But he maintains a centrist image that has had strong appeal in a conservative-leaning district — which includes Kansas City suburbs and the state capital of Jefferson City — that gave 64 percent of its votes to President George W. Bush in 2004 and 61 percent to Republican John McCain in 2008.

Only 10 districts currently held by House Democrats gave McCain a greater share of the vote than Missouri’s 4th, according to calculations by CQ Politics.

Skelton also has held the prestigious chairmanship of the House Armed Services Committee since the Democrats gained the House majority in the 2006 elections, after serving as ranking Democrat for the eight years prior.

But it appears either Stouffer or Hartzler, as experienced candidates, would start out a general election campaign on firmer footing than Skelton’s 2008 opponent, tire store owner Jeff Parnell, who raised just $4,000 to the incumbent’s $1.1 million during that election cycle.

Both of the contenders for the August 2010 Republican primary have appeared on ballots multiple times. Stouffer was elected to the Missouri Senate in 2004 and re-elected in 2008, while Hartzler won state House races in 1994, 1996 and 1998.